Macroautophagy, the major lysosomal degradative pathway in cells, is responsible for degrading long-lived cytoplasmic constituents and is the principal mechanism for turning over cellular organelles and protein aggregates too large to be degraded by the proteasome (Klionsky, 2007; Mizushima, 2007; Rubinsztein, 2006). When macroautophagy is genetically ablated, neurons accumulate ubiquitinated protein aggregates and degenerate (Hara et al., 2006; Komatsu et al., 2006).
Macroautophagy, hereafter referred to as autophagy, involves the sequestration of a region of cytoplasm within an enveloping double-membrane structure to form an autophagosome. Autophagosome formation is induced by inhibition of mTOR (mammalian target of Rapamycin), a protein kinase modulated by signaling pathways involving either class I phosphatidylinositol-3-kinase (PI3K)-Akt/protein kinase B (PKB) (Schmelzle and Hall, 2000) or AMP-activated protein kinase (AMPK) (Samari and Seglen, 1998). Autophagosomes and their contents are cleared upon fusing with late endosomes or lysosomes that contain cathepsins, other acid hydrolases, and vacuolar [H+] ATPase (v-ATPase) (Yamamoto et al., 1998), a proton pump that acidifies the newly created autolysosome. Acidification of autolysosomes is crucial for activating cathepsins and effecting proteolysis of substrates; however, these late digestive steps of autophagy remain relatively uncharacterized.
Autophagic vacuoles (AVs), the general term for intermediate vesicular compartments in the process of autophagy, accumulate in several neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease, and Huntington disease (Cuervo et al., 2004; Nixon et al., 2005; Ravikumar et al., 2004). Autophagy pathology is exceptionally robust in AD, where AVs collect in massive numbers within grossly distended portions of axons and dendrites of affected neurons (Yu et al., 2005), likely reflecting defective AV clearance (Boland et al., 2008). This lysosome-related pathology, along with neuronal loss and amyloid deposition, are greatly accentuated in early-onset familial AD (FAD) due to mutations of PS1, the most common cause of FAD (Cataldo et al., 2004).
Presenilin-1 (PS1), a ubiquitous transmembrane protein, has diverse putative biological roles in cell adhesion, apoptosis, neurite outgrowth, calcium homeostasis, and synaptic plasticity (Kim and Tanzi, 1997; Shen and Kelleher, 2007). PS1 holoprotein, a ˜45 kDa protein, is cleaved in the endoplasmic reticulum (ER) to create a heterodimer (Zhang et al., 1998). Many known PS1 functions, but not all, involve the cleaved heterodimeric form of PS1 as the catalytic subunit of the gamma (γ)-secretase enzyme complex, which mediates the intramembranous cleavage of many type 1 membrane proteins, including APP and Notch (Citron et al., 1997; De Strooper et al., 1998). Although the pathogenic effects of PS1 mutations in AD are commonly ascribed to increased generation of the neurotoxic Aβ peptide from APP, not all of the disease-causing PS1 mutations have this effect (Junichi et al., 2007). Additional contributions to AD pathogenesis may involve loss of one or more of the other suspected biological functions of PS1, which include the roles described above and the trafficking or turnover of transmembrane proteins (Naruse et al., 1998).
The underlying causes of AD are not understood. It is to this objective of better defining the mechanistic underpinnings of AD etiology and/or progression that the present results are directed. An appreciation of the biochemical and cellular mechanisms that contribute to AD, in turn, imparts guidance that can be applied to the development and testing of therapeutics for the treatment of AD patients.
The citation of references herein shall not be construed as an admission that such is prior art to the present invention.